Achieving (bio)macromolecular structural assignment from the interpretation of ion mobility spectrometry (IMS) experiments
requires successful comparison with computer modeling. Replica-exchange molecular dynamics simulations with suitable force
fields not only offer a convenient framework to locate relevant conformations, especially in the case of multiple-funnel energy
landscapes, but they are also well suited to statistical analyses. In the present paper, we discuss two extensions of the
method used to improve its efficiency in the context of IMS. Two doubly-protonated polyalanines [RA4XA4K + 2H]2+ with X = V and D appear as favorable cases for which the calculated collision cross-section distributions naturally agree
with the measurements, providing reliable candidate structures. For these compounds, a careful consideration of other order
parameters based on the weighted histogram method resolves several otherwise hidden underlying conformational families. In
the case of a much larger peptide exhibiting bistability, assignment is more difficult but could be achieved by guiding the
sampling with an umbrella potential using the square gyration radius as the biasing coordinate. Applied to triply protonated
bradykinine, the two presented methods indicate that different conformations compatible with the measurements are very close
in energy. 相似文献
Trapped ion mobility spectrometry (TIMS) is a new high resolution (R up to ~300) separation technique that utilizes an electric field to hold ions stationary against a moving gas. Recently, an analytical model for TIMS was derived and, in part, experimentally verified. A central, but not yet fully explored, component of the model involves the fluid dynamics at work. The present study characterizes the fluid dynamics in TIMS using simulations and ion mobility experiments. Results indicate that subsonic laminar flow develops in the analyzer, with pressure-dependent gas velocities between ~120 and 170 m/s measured at the position of ion elution. One of the key philosophical questions addressed is: how can mobility be measured in a dynamic system wherein the gas is expanding and its velocity is changing? We noted previously that the analytically useful work is primarily done on ions as they traverse the electric field gradient plateau in the analyzer. In the present work, we show that the position-dependent change in gas velocity on the plateau is balanced by a change in pressure and temperature, ultimately resulting in near position-independent drag force. That the drag force, and related variables, are nearly constant allows for the use of relatively simple equations to describe TIMS behavior. Nonetheless, we derive a more comprehensive model, which accounts for the spatial dependence of the flow variables. Experimental resolving power trends were found to be in close agreement with the theoretical dependence of the drag force, thus validating another principal component of TIMS theory.
A novel concept for ion spatial peak compression is described, and discussed primarily in the context of ion mobility spectrometry (IMS). Using theoretical and numerical methods, the effects of using non-constant (e.g., linearly varying) electric fields on ion distributions (e.g., an ion mobility peak) is evaluated both in the physical and temporal domains. The application of a linearly decreasing electric field in conjunction with conventional drift field arrangements is shown to lead to a reduction in IMS physical peak width. When multiple ion packets (i.e., peaks) in a selected mobility window are simultaneously subjected to such fields, there is ion packet compression (i.e., a reduction in peak widths for all species). This peak compression occurs with only a modest reduction of resolution, which can be quickly recovered as ions drift in a constant field after the compression event. Compression also yields a significant increase in peak intensities. Ion mobility peak compression can be particularly useful for mitigating diffusion-driven peak broadening over very long path length separations (e.g., in cyclic multi-pass arrangements), and for achieving higher S/N and IMS resolution over a selected mobility range.
The gas-phase conformations of electrosprayed ions of the model peptide KKDDDDIIKIIK have been examined by ion mobility spectrometry (IMS) and hydrogen deuterium exchange (HDX)-tandem mass spectrometry (MS/MS) techniques. [M+4H]4+ ions exhibit two conformers with collision cross sections of 418 Å2 and 471 Å2. [M+3H]3+ ions exhibit a predominant conformer with a collision cross section of 340 Å2 as well as an unresolved conformer (shoulder) with a collision cross section of ~367 Å2. Maximum HDX levels for the more compact [M+4H]4+ ions and the compact and partially-folded [M+3H]3+ ions are ~12.9, ~15.5, and ~14.9, respectively. Ion structures obtained from molecular dynamics simulations (MDS) suggest that this ordering of HDX level results from increased charge-site/exchange-site density for the more compact ions of lower charge. Additionally, a new model that includes two distance calculations (charge site to carbonyl group and carbonyl group to exchange site) for the computer-generated structures is shown to better correlate to the experimentally determined per-residue deuterium uptake. Future comparisons of IMS-HDX-MS data with structures obtained from MDS are discussed with respect to novel experiments that will reveal the HDX rates of individual residues. Graphical Abstract
High field asymmetric wave ion mobility spectrometry (FAIMS) is a powerful tool to detect and characterize gas-phase ions, while the unsolvable partial differential equation of ions moving in ion drift tube poses a big challenge to FAIMS spectral peak analysis. In this work, a universal and effective model of FAIMS spectral peak profile has been proposed by introducing ion trajectory and loss height. With this model, the influence of the structure of ion drift tube, dispersion voltages, compensation voltages, and carrier gas flow rate on the FAIMS spectral peak characteristics like peak shape, full width at half maximum and peak height is analyzed and discussed. The results show that the influence of different factors on the FAIMS spectral peak profile can be qualitatively described by the model which agrees with the experimental data. 相似文献
Ion mobility spectra for ten alcohols have been studied in an ion mobility spectrometry apparatus equipped with a corona discharge ionization source. Using protonated water cluster ions as the reactant ions and clean air as the drift gas, the alcohols exhibit different product ion characteristic peaks in their ion mobility spectra. The detection limit for these alcohols is at low concentration pmol/L level according to the concentration calibration by exponential dilution method. Based on the measured ion mobilities, several chemical physics parameters of the ion-molecular interaction at atmosphere were obtained, including the ionic collision cross sections, diffusion coefficients, collision rate constants, and the ionic radii under the hard-sphere model approximation. 相似文献
Abstract Positive ion mobility spectra of different pesticides, such as sevin or carbaryl (1‐napthyl methylcarbamate), amitraz (N,N′‐[(methylimino) dimethylidyne]di‐2,4‐xylidine), and metalaxyl (methyl‐N‐(2,6‐dimethylphenyl)‐N‐(2‐xylyl)‐DL‐alaninate) as carbamate, amidine, and alkaline groups, respectively, have been studied in air at ambient pressure using ion mobility spectrometry method with 63Ni ionization source. The limits of detection (LODs) were 5.3×10?10, 5.8×10?10, and 4.5×10?10 g for sevin, amitraz, and metalaxyl, respectively. The working range of these compounds was about three orders of magnitude and the relative standard deviation (RSD) of repeatability at the 5 µg mL?1 level were all below 14%. Furthermore, in this study, the influences of IMS cell temperature on the ion mobility spectra of these compounds were investigated. 相似文献
Journal of Analytical Chemistry - The drift time and ion mobility of imidazole are determined and a procedure for the mathematical processing of spectra is developed. The specific features of... 相似文献
Correlations between the dimensions of a 2-D separation create trend lines that depend on structural or chemical characteristics
of the compound class and thus facilitate classification of unknowns. This broadly applies to conventional ion mobility spectrometry
(IMS)/mass spectrometry (MS), where the major biomolecular classes (e.g., lipids, peptides, nucleotides) occupy different
trend line domains. However, strong correlation between the IMS and MS separations for ions of same charge has impeded finer
distinctions. Differential IMS (or FAIMS) is generally less correlated to MS and thus could separate those domains better.
We report the first observation of chemical class separation by trend lines using FAIMS, here for lipids. For lipids, FAIMS
is indeed more independent of MS than conventional IMS, and subclasses (such as phospho-, glycero-, or sphingolipids) form
distinct, often non-overlapping domains. Even finer categories with different functional groups or degrees of unsaturation
are often separated. As expected, resolution improves in He-rich gases: at 70% He, glycerolipid isomers with different fatty
acid positions can be resolved. These results open the door for application of FAIMS to lipids, particularly in shotgun lipidomics
and targeted analyses of bioactive lipids. 相似文献
Abstract Automated, continuous monitoring of organic vapors in air under three field designs for plume drift was demonstrated using a hand-held ion mobility spectrometer (IMS) in characterizing IMS behavior as a point sensor. In one field study, the IMS was placed 50cm from a 9m2 grass plot contaminated with methylsalicylate and response to airborne vapors was recorded during a 13hr period of atmospheric turbulence to illustrate susceptibility of point sensors to wind direction. A similar study under near-quiescent atmospheric conditions was made using dimethylsulfoxide. In a third study, the plume from a point source of dipropyleneglycolmonomethylether was interrogated over a 25m × 12m grid downwind with windspeeds of 6–18km h?. Laboratory studies were used to measure instrumental response times and influences from potentially interfering atmospheric organic pollutants. 相似文献
Differential ion mobility spectrometry (DIMS) has the ability to separate gas phase ions based on their difference in ion mobility in low and high electric fields. DIMS can be used to separate mixtures of isobaric and isomeric species indistinguishable by mass spectrometry (MS). DIMS can also be used as a filter to improve the signal-to-background of analytes in complex samples. The resolving power of DIMS separations can be improved several ways, including increasing the dispersion field and increasing the amount of helium in the nitrogen carrier gas. It has been previously demonstrated that the addition of helium to the DIMS carrier gas provides improves separations when the dispersion field is the kept constant as helium content is varied. However, helium has a lower breakdown voltage than nitrogen. Therefore, as the percent helium content in the nitrogen carrier gas is increased, the highest dispersion field accessible decreases. This work presents the trade-offs between increasing dispersion fields and using helium in the carrier gas by comparing the separation of a mixture of isobaric peptides. The maximum resolution for a separation of a mixture of three peptides with the same nominal molar mass was achieved by using a high dispersion field (~72 kV/cm) with pure nitrogen as the carrier gas within the DIMS assembly. The conditions used to achieve the maximum resolution also exhibit the lowest ion transmission through the assembly, suggesting that it is necessary to consider the trade-off between sensitivity and resolution when optimizing DIMS conditions for a given application. Figure
Miniaturised field asymmetric waveform ion mobility spectrometry (FAIMS), combined with mass spectrometry (MS), has been applied to the study of self-assembling, noncovalent supramolecular complexes of 3-methylxanthine (3-MX) in the gas phase. 3-MX forms stable tetrameric complexes around an alkali metal (Na+, K+) or ammonium cation, to generate a diverse array of complexes with single and multiple charge states. Complexes of (3-MX)n observed include: singly charged complexes where n = 1–8 and 12 and doubly charged complexes where n = 12–24. The most intense ions are those associated with multiples of tetrameric units, where n = 4, 8, 12, 16, 20, 24. The effect of dispersion field on the ion intensities of the self-assembled complexes indicates some fragmentation of higher order complexes within the FAIMS electrodes (in-FAIMS dissociation), as well as in-source collision induced dissociation within the mass spectrometer. FAIMS-MS enables charge state separation of supramolecular complexes of 3-MX and is shown to be capable of separating species with overlapping mass-to-charge ratios. FAIMS selected transmission also results in an improvement in signal-to-noise ratio for low intensity complexes and enables the visualization of species undetectable without FAIMS.
Ion mobility measurements of product ions were used to characterize the collisional cross section (CCS) of various complex lipid [M-H]? ions using traveling wave ion mobility mass spectrometry (TWIMS). TWIMS analysis of various product ions derived after collisional activation of mono- and dihydroxy arachidonate metabolites was found to be more complex than the analysis of intact molecular ions and provided some insight into molecular mechanisms involved in product ion formation. The CCS observed for the molecular ion [M-H]? and certain product ions were consistent with a folded ion structure, the latter predicted by the proposed mechanisms of product ion formation. Unexpectedly, product ions from [M-H-H2O-CO2]? and [M-H-H2O]? displayed complex ion mobility profiles suggesting multiple mechanisms of ion formation. The [M-H-H2O]? ion from LTB4 was studied in more detail using both nitrogen and helium as the drift gas in the ion mobility cell. One population of [M-H-H2O]? product ions from LTB4 was consistent with formation of covalent ring structures, while the ions displaying a higher CCS were consistent with a more open-chain structure. Using molecular dynamics and theoretical CCS calculations, energy minimized structures of those product ions with the open-chain structures were found to have a higher CCS than a folded molecular ion structure. The measurement of product ion mobility can be an additional and unique signature of eicosanoids measured by LC-MS/MS techniques.
When utilized in conjunction with modeling, the collision cross section (Ω) from ion mobility spectrometry can be used to
deduce the gas phase structures of analyte ions. Gas phase conformations are determined computationally, and their Ω calculated
using an approximate method, the results of which are compared with experimental data. Though prior work has focused upon
rigid small molecules or large biomolecules, correlation of computational and experimental Ω has not been thoroughly examined
for analytes with intermediate conformational flexibility, which constitute a large fraction of the molecules studied in the
field. Here, the computational paradigm for calculating Ω has been tested for the tripeptides WGY, YGW, and YWG (Y = tyrosine,
W = tryptophan, G = glycine). Experimental data indicate that Ωexp (YWG) > Ωexp (WGY) ≈ Ωexp (YGW). The energy distributions of conformations obtained from tiers of simulated annealing molecular dynamics (SAMD) were
analyzed using a wide array of density functionals. These quantum mechanical energy distributions do not agree with the MD
data, which leads to structural differences between the SAMD and DFT conformations. The latter structures are obtained by
reoptimization of the SAMD geometries, and are the only suite of structures that reproduce the experimental trend in analyte
separability. In the absence of fitting Lennard Jones potentials that reproduce experimental results for the Trajectory Method,
the Exact Hard Sphere Scattering method produced numerical values that are in best agreement with the experimental cross sections
obtained in He drift gas. 相似文献
Ion mobility spectrometry-mass spectrometry (IMS-MS) techniques are used to study the general effects of phosphorylation on peptide structure. Cross sections for a library of 66 singly phosphorylated peptide ions from 33 pairs of positional isomers, and unmodified analogues were measured. Intrinsic size parameters (ISPs) derived from these measurements yield calculated collision cross sections for 85% of these phosphopeptide sequences that are within ±2.5% of experimental values. The average ISP for the phosphoryl group (0.64 ± 0.05) suggests that in general this moiety forms intramolecular interactions with the neighboring residues and peptide backbone, resulting in relatively compact structures. We assess the capability of ion mobility to separate positional isomers (i.e., peptide sequences that differ only in the location of the modification) and find that more than half of the isomeric pairs have >1% difference in collision cross section. Phosphorylation is also found to influence populations of structures that differ in the cis/trans orientation of Xaa–Pro peptide bonds. Several sequences with phosphorylated Ser or Thr residues located N-terminally adjacent to Pro residues show fewer conformations compared to the unmodified sequences.
